Implant comprising a two-piece joint

ABSTRACT

An implant, in particular an intervertebral implant, comprising (A) two articulating parts ( 4; 5 ) having each a central axis ( 1; 26 ), each a curved slide surface ( 6; 7 ) intersecting the central axes ( 1; 26 ), and each an axially outermost end ( 14; 15 ) that can be connected to a bone, where (B) the slide surfaces ( 6; 7 ) are curved and displaceable one on the other, where (D) the second articulating part ( 5 ) is rotatable about two mutually skewed axes of rotation ( 10; 11 ) relative to the first articulating part ( 4 ).

The present invention relates to an implant, in particular anintervertebral implant defined in the preamble of claim 1.

As regards individual human body joints for instance the joint betweenthe distal femur condyle and the patella, or between the metacarpaliaand the distal phalange, there are articulation surfaces allowing apreferred articulation about one or several joint axes while on theother hand restricting articulation about all other spatially possibleaxes or rotation or precluding them entirely. Implants having a joint ofseveral axes of rotation on the other hand also are used asintervertebral implants or intervertebral disk endoprostheses.

Conventionally one or more damaged natural vertebral disks, also adamaged vertebral disk nucleus, will be removed and be partly replacedby implants or prostheses inserted into the intervertebral space betweenthe two adjacent vertebras. The purpose is to restore conditions thatshall be as natural as possible, that is, in particular the originalintervertebral disk height and hence the original spacing between thetwo adjacent vertebras. Hopefully the vertebras' displacements takeplace henceforth within their natural range of excursions without beingdegraded by the implant or prosthesis. Such requirement entailsachieving displaceabilities within the natural limits on leaning forwardor backward, namely bending or stretching the spinal column as well aslaterally bending the vertebras. Also a spatial change in position ofthe adjacent vertebras relative to one another shall be feasible withinthe ranges of physiological displacements.

U.S. Pat. No. 6,368,350 (Erickson) discloses an implant replacing avertebral disk. This known implant substantially includes twoarticulating parts with end plates. In one embodiment mode, this jointconsisting of a convex articulating part and of a complementary concavearticulating part comprises spherical surfaces of articulation, as aresult of which the articulating parts may be rotated about differentaxes of rotation that are situated in one plane, no restriction on anaxis of rotation of lateral vertebral bending and on a further axis ofrotation for vertebral bending/stretching taking place. Moreoverrotation of the adjoining vertebras about their longitudinal axis ispossible in unrestricted manner. In another embodiment mode, thearticulation surfaces are the surfaces of an ellipsoid, as a result ofwhich the axes of rotation again will not be restricted to an axis ofrotation of lateral vertebral bending and to a further axis of rotationof bending/stretching the vertebras.

The object of the present invention is palliation of the above state ofthe art. The objective is to create an implant having two articulatingarticulating parts, said implant comprising two axes of rotationconfigured skewed to each other and spatially a defined distance apart,as a result of which, following resection of the in-between vertebraldisk, the displacements of the adjacent vertebras shall be reproduced.

The present invention solves the above problem by means of an implant,preferably an intervertebral implant exhibiting the features of claim 1.

Essentially the implant of the present invention comprises twoarticulating parts with mutually articulating slide surfaces that, uponrotation of the articulating parts, move relative to each other. Eacharticulating part comprises a central axis substantially parallel to orcoaxial with to the longitudinal axes of two adjoining bones, saidlongitudinal and central axes coinciding in the rest position when thebody is kept correspondingly erect, that is when the articulating partsare not mutually oblique. Moreover the articulating parts each comprisean axially outermost end that can be connected to a bone. When theimplant is designed in the form of an intervertebral implant, said endscan be connected to the adjoining vertebras. The slide surfacesintersect the central axes of the articulating parts and will moverelative to each other when the articulating parts rotate, the secondarticulating part being rotatable about two mutually skewed axes ofrotation relative to the first articulating part.

In the following discussion, the first articulating part is assumedstationary and the second articulating part is assumed displaceable. Asa result other axes of rotation are displaceable relative to the firstarticulating part and are stationary relative to the second articulatingpart.

Essentially the following advantages are offered by the implant, inparticular the intervertebral implant of the present invention;

-   -   the position of the center of rotation during bending or        stretching and/or when laterally bending the vertebras is better        matched to physiology,    -   the slide surfaces can be displaced relative to and on each        other in the absence of substantial friction, and forces and        torques are minimized as much as possible by reproducing the        corresponding lever arms, and    -   twisting motions about the spinal column's longitudinal axis may        be opposed by bracing.

The slide surfaces of the preferred embodiment of the implant of theinvention are saddle-shaped and comprise each a saddle point. Thesaddle-shaped slide surfaces are designed in a manner that two surfacepoints may be found in the vicinity of a surface point P bounded by thearticulating part's dimensions so that said surface points shall besituated on different sides of the tangential plane through the surfacepoint P.

In a further embodiment mode of the embodiment mode of the invention,the axes of rotation cross one another at angle preferably between 80and 100°. This feature offers the advantage that, illustratively, anintervertebral implant can be implanted in a manner that one of the axesof rotation runs parallel to or coaxially with axis of the laterallybending vertebras and the other axis of rotation runs parallel to orcoaxially with the axis of vertebral bending or stretching.

In another embodiment of the implant of the invention, the axes ofrotation are apart by a minimum distance A between 0.1 and 20 mm,preferably between 2 and 20 mm. In part the distance A is alsodetermined by the site of application of the implant of the invention atthe spinal column and it depends according to segment height in thelumbar spinal column, decreasing in the direction of the thoracicvertebras.

Preferably the slide surfaces are designed in a manner that uponrotation of the second articulating part about each of the axes ofrotation, the second saddle point moves along the arc of circleconcentric with the axes of rotation. The saddle-shaped design of theslide surfaces offers the advantage of also allowing rotating thearticulating parts about the central axis of these parts. However, whenthe articulating parts rotate axially relative to each other, said twoarticulating parts move axially away from one another, and as a resultaxial rotation is possible only if there is simultaneous change inimplant height. Because of the constant prestressing force in theligaments, muscles and sinews in the spinal column, the articulatingparts can rotate axially only to a limited degree. This feature appliessimilarly in the lumbar spinal column to the physiological case onaccount of the rotation of the rear elements such as facetted joints,that allow only limited axial rotation.

Viewed in the rest condition of the articulating parts, the slidesurfaces preferably shall be congruent. By designing the implant of theinvention to have congruent slide surfaces, this invention precludesvertebral twisting about the spinal column's longitudinal directionunless there be a change in the height of the intervertebral implant.Changing the intervertebral implant height during such a displacement,the ligaments will be tensioned, resulting in opposition to vertebraltwisting.

In a further embodiment mode of the implant of the present invention,each of the outermost ends of the articulating parts comprises aconnection element that can be connected to the adjoining bone orvertebra. When the implant of the present invention is an intervertebralimplant, said connection elements preferably are cover plates eachhaving an axially outermost surface transverse to the central axes, saidsurfaces being fitted with a three-dimensional topography, for instanceserrations or fins.

In this design furthermore one of the cover plates may be integral withthe adjoining articulating part.

In one embodiment mode of the implant of the present invention in theform of an intervertebral implant, one of the cover plates comprises aguide, or groove, perpendicular to the central axis of the adjoiningarticulating part and insertable into the adjoining articulating part'srear end which is complementary to said guide. This feature offers theadvantage first that the two cover plates resting against the firstarticulating part may jointly be moved between the adjacent vertebras,next that the over plates can be pressed against the end surfaces of theadjoining vertebras and that only in the end the second articulatingpart may be inserted between the first articulating part and the secondcover plate, as a result of which the vertebras only need be underminimal traction during implantation. The second, inserted, articulatingpart is affixed, following insertion, to the second cover plate.

Depending on application, the articulating parts may be metal/metalpairs. Moreover ceramics also may be appropriately used, shocks willonly slightly load the articulating parts because of the highprestressing forces in the spinal column between the adjoiningvertebras.

In a further embodiment mode of the implant of the present invention,one of the articulating parts is made of plastic, as a result of which:

-   -   known performance-tested combinations of joint replacement        materials such as highly crosslinked polyethylene (UHMWPE        [ultra-high molecular weight polyethylene]) and a        cobalt/chromium alloy may be used,    -   low friction when the slide surfaces are moved relative to each        other can be attained, and    -   the rotations of the articulating parts about the central axes        can be damped.

In yet another embodiment mode of the implant of the present invention,one of the articulating parts may be received at the pertinentconnection element, respectively the pertinent cover plate, so as to berotatable about its central axis. Illustratively the outermost end ofthe articulating part may rest in a complementary recess coaxial withthe central axis and at the pertinent connection element, i.e. theassociated cover plate. Conversely the connecting element may be fittedwith an elevation coaxial with the central axis and the articulatingpart may be fitted with a recess. In this manner the invention offersthe advantage that twisting motions of the two adjacent vertebras shallnot be prevented by the implant.

In another embodiment mode of the implant of the present invention, oneof the articulating parts received at the associated connecting element,i.e. the associated cover plate in a manner to be displaceable parallelto the displacement axis which is parallel to the central axis.Preferably the outermost articulating part end is terminally fitted witha widening configured coaxially with the central axis, whereas theassociated connection element, i.e. the associated cover plate, enclosesa recess which is complementary to the articulating part's outermost endand which is fitted with a recess to receive said widening. This designof the implant of the present invention also allows one-axis shearingmotions between the two vertebras adjoining the implant which does nothamper such motions. The vertebral shearing motion can be restricted bycontrolling the length of said cavity.

In yet another embodiment mode of the implant of the present invention,one of the articulating parts is received at the associated connectionelement, i.e. the associated cover plate, so as to be displaceable in aplane perpendicular to the central axis. Preferably the outermost end ofthe articulating part then comprises a smaller diameter than the recessat the corresponding connection element. This feature offers theadvantage that shearing motions of the vertebras adjoining the implantalso are feasible relative to several axes.

Further advantageous embodiment modes of the present invention aredefined in the dependent claims.

The invention, and further developments of the invention, are elucidatedbelow in relation to the partly schematic drawings of severalillustrative embodiments.

FIG. 1 a is an elevation of an embodiment mode of the present implant,the second articulating part being rotated about the first axis ofrotation,

FIG. 1 b is a sideview of the embodiment of the implant of the inventionshown in FIG. 1 a,

FIG. 1 c is a view of the embodiment of the implant of the inventionshown in FIGS. 1 a and 1 b, the second articulating part having beenrotated about the second axis of rotation,

FIG. 1 d is a sideview of the embodiment of the implant of the inventionshown FIG. 1 c,

FIG. 2 is a perspective view of the articulating part of an embodimentof the implant of the invention,

FIG. 3 is a perspective of an embodiment of the implant of the inventionin the form of an intervertebral implant,

FIG. 4 a is a ventral view of an embodiment mode of the implant of theinvention,

FIG. 4 b is a lateral view of the embodiment mode of the implant of theinvention of FIG. 4 a,

FIG. 5 is a view of a further embodiment mode of the implant of theinvention having an articulating part mounted on the cover plate androtatable about the central axis,

FIG. 6 a is a view of another embodiment mode of the implant of theinvention having an articulating part displaceable in a plane verticalto the central axis,

FIG. 6 b is a section of the embodiment mode of the implant of theinvention shown in FIG. 6 a,

FIG. 7 a is a view of a further embodiment mode of the implant of theinvention having an articulating part displaceable perpendicularly tothe central axis, and

FIG. 7 b is a section of the embodiment mode of the implant of theinvention shown in FIG. 7 a.

FIGS. 1 a through 1 d show an embodiment of the implant of the inventioncomprising a first and a second articulating joint 4; 5, FIG. 1 bshowing the two articulating parts 4;5 in sideview A. The firstarticulating part 4 comprises a first central axis 1 and a first slidesurface 6 intersecting said first central axis 1. Said first slidesurface 6 is saddle-shaped and has a first saddle point 8. Similarly tothe case of the first articulating part 4, the second articulating part5 has a second central axis 26 and a second slide surface 7 intersectingthis second central axis 26. The second slide surface 7 also issaddle-shaped and includes the saddle point 9. Moreover the articulatingparts 4; 5 comprise outermost ends 14; 15 which may be made to restagainst the end surfaces of bones adjoining the articulating parts, inparticular adjoining vertebras. The projections of the axes of rotation10; 11 intersect in a plane orthogonal to the central axis 1 at an angleof 90°. In this design of the slide surfaces 6; 7, the minimal distanceA is vertical to the two axes of rotation 10; 11. In FIGS. 1 a and 1 b,the second articulating part 5 has been rotated about the first axes ofrotation 10 (FIG. 1 b). The second saddle point 9 is displaced duringthe rotation of the second articulating part 5 by the angle β along anarc of circle of radius R₁ and concentric with the first axis ofrotation 10.

FIGS. 1 c and 1 d show the articulating parts 4; 5 of the embodimentshown in FIGS. 1 a and 1 b, the second articulating part 5 having beenrotated about the second axis of rotation 11 (FIG. 1 c) by the angle α.During this rotation of the articulating part 5 about the second axis ofrotation 11, the second saddle point 9 is displaced along an arc ofcircle having a radius R₂ concentric with the second axis of rotation11.

FIG. 2 shows the first articulating part 4 having a saddle-shaped slidesurface 6. The two axes of rotation 10; 11 are shown in the restcondition of the implant of the invention at corresponding bodyerectness, that is the central axis 1 of the first articulating part 4coincides with the central axis 26 of the second articulating part 5(FIG. 1). In the rest position, the axes of rotation 10; 11 areperpendicular to the central axis 1 and lie in the planes 22; 23. Thefirst axis of rotation 10 runs perpendicularly to a first plane 22 whichis defined by the central axis 1 and the second axis of rotation 11. Thesecond axis of rotation 11 runs perpendicularly to a second plane 23itself perpendicular to the first plane 22 and defined by the centralaxis 1 and the first axis of rotation 10. In the rest position, thefirst saddle point 8 of the first slide surface 6 is situated both onthe central axis 1 and on two arcs or circle 24; 25 of which the centerin the rest position of the articulating parts 4; 5 is defined by theintersection of the central axis 1 with each of the axes of rotation 10;11. The first slide surface 6 on one hand tightly follows the first arcof circle 24 concentric with the first axis of rotation 10 and on theother hand also the second arc of circle 25 concentric with the axis ofrotation 11. The radii R₁ of the first arc of circle 24 and R₂ of thesecond arc of circle 25 are equal when the slide surfaces 7; 8 arecongruent and correspond to half the distance A between the two axes ofrotation 10; 11.

The slide surface is designed in a manner to be the complementary shapeof a patch of a toroidal surface.

FIG. 3 shows an embodiment of the implant of the invention in the formof an intervertebral implant. At their outermost ends 14; 15, the twoarticulating joints 4; 5 comprise a connection site 2; 3 for each coverplate 12; 13 having surfaces 16; 17 that are axially outermost along thecentral axes 1; 26 and that are configured transversely to the centralaxes 1; 26, said surfaces 16; 17 being movable to rest against thevertebras' end surfaces. The cover plates 12; 13 each comprise twolateral edge surfaces 27, one anterior edge surface 28 and one posterioredge surface 29, the lateral edge surfaces 27 being substantiallyparallel to the first axis of rotation 10. Both the anterior and theposterior edge surfaces 28; 29 are configured substantially parallel tothe second axis of rotation 11. The implant is configured between the(omitted) adjoining vertebras in a manner that rotating the articulatingparts 4; 5 about the first axis of rotation 10 allows laterally bendingthe vertebras connected to the implant, whereas rotating thearticulating parts 4; 5 about the second axis of rotation 11 allowsbending, respectively stretching the vertebras connected to the implant.In the embodiment of the implant of the invention being discussed here,the first cover plate 12 is rigidly joined to the first articulatingpart 4 whereas the second cover plate 13 comprises a guide 20 in theform of a channel which runs perpendicularly to the central axis 26 andsubstantially parallel to the lateral edge surfaces 27, as a result ofwhich the second articulating part 5 together with its outermost end 15designed to be complementary to the guide 20 can be inserted into thisguide. Moreover serrations/fins 19 are present at the outermost surfaces16; 17 of the cover plates 12; 13 to provide the primary implantstabilization at the adjoining vertebras.

The embodiment of the implant of the invention shown in FIGS. 4 a and 4b differs from the embodiment of the implant of the invention shown inFIG. 3 only in that it is devoid of the serrations/fins 19 and in thatthe articulating parts 4; 5 are rotatable only within limited angles ofrotation α and β about the axes of rotation 10; 11. FIG. 4 a shows theimplant ventrally, that is parallel to the axis of rotation 10, and FIG.4 b shows the implant laterally, that is parallel to the axis ofrotation 11 (FIG. 4 b). The limitation of the angles of rotation α and βis determined by selecting the sizes H_(L); H_(A); H_(P); R₁; R₂; h₁ andh₂ at the articulating parts 4; 5, where when the maximum angles ofrotation α or β are reached, the particular ends 32; 33; 34—of thearticulating parts 4; 5—pointing toward the other articulating part 4; 5will rest on the inner surfaces 30; 31 of the cover plate 12; 13opposite the articulating part 4; 5, and:

-   -   H_(L) is the height between the first saddle point 8 and the        inner ends 32 of the first articulating joint 4 that point        toward the second articulating joint 5;    -   H_(A) is the height between the second saddle point 9 and the        anterior end 34 of the articulating part 5 pointing toward the        first articulating part 4;    -   H_(P) is the height between the saddle point 9 and the posterior        end 33 of the second articulating part 5 pointing toward the        first articulating part 4;    -   R₁ is the radius of the first slide surface 6 in the first plane        22 (FIG. 2) perpendicular to the first axis of rotation 10 and        containing the first saddle point 8;    -   R₂ is the radius of the second slide surface in the second plane        23 (FIG. 2) perpendicular to the second axis of rotation 11 and        containing the second saddle point 9;    -   h₁ is the height between the first saddle point 8 and the inner        surface 31 of the first cover plate 12; and    -   h₂ is the height between the second saddle point 9 and the inner        surface 30 of the second cover plate 13.

FIG. 5 shows an embodiment of the implant of the invention differingfrom the embodiments shown in FIGS. 1 through 4 in that the outermostend 14 of the first articulating part 4 is received in a complementaryrecess 37 in the cover plate 12 coaxial with the central axis as aresult of which the first articulating part 4 may be rotated about thecentral axis 1 to be assembled to the cover plate 12.

FIGS. 6 a and 6 b show an embodiment of the implant of the inventionwhich differs from the embodiment shown in FIG. 5 only in that therecess 37 exhibits a larger diameter toward the central axis 1 than theoutermost end 14 of the first articulating part 4, as a result of whichthe first articulating part 4 is displaceable relative to the coverplate 12 in a plane that is perpendicular to the central axis 1.

FIGS. 7 a and 7 b show an embodiment of the implant of the inventionwhich differs from the embodiment shown in FIG. 5 only in that therecess 37 is oval parallelly to a displacement axis 40 which isperpendicular to the central axis 1 and comprises a cavity 39 whereasthe outermost end 14 of the first articulating part 4 terminallycomprises a widening 38 coaxial with the central axis 1 entering saidcavity 39, as result of which the first articulating part 4 on one handis displaceable parallel to the displacement axis 40 and on the otherhand is axially secured in position relative to the central axis 1 bythe widening 38 engaging the cavity 39.

1. An intervertebral implant for implantation between first and secondvertebra, said implant comprising: a first end plate having an innerside and a first bone contacting surface, said first bone contactingsurface being sized and configured to contact said first vertebra; asecond end plate having an inner side and a second bone contactingsurface, said second bone contacting surface being sized and configuredto contact said second vertebra; a first member having a first end and asecond end, said first end being sized and configured to contact saidfirst end plate, said second end having a first saddle-shaped contactsurface having a compound radius with at least two inflection points;and a second member having a first end and a second end, said first endbeing sized and configured to contact said second end plate, said secondend having a second saddle-shaped contact surface for contacting saidfirst saddle-shaped contact surface of said first member, said first andsecond saddle-shaped contact surfaces being sized and configured topermit said first member to articulate with respect to said secondmember along said first and second saddle-shaped contact surfaces;wherein said inner side of said first end plate includes a recess, therecess having a diameter and said first end of said first member has adiameter, the diameter of the recess being larger than the diameter ofthe first end of the first member so that said first member isreceivable within said recess and moveable with respect to said firstend plate even after implantation.
 2. The implant of claim 1, whereinsaid first member is slidably displaceable with said first end plate. 3.The implant of claim 2, wherein said first member is sized andconfigured to be slidably displaced in a first direction but not in asecond direction.
 4. The implant of claim 3, wherein said recess formedin said inner side of said first member is an oval recess.
 5. Theimplant of claim 1, wherein said first member is permitted to twist withrespect to said first end plate.
 6. The implant of claim 1, wherein saidfirst member is permitted to rotate with respect to said first endplate.
 7. The implant of claim 1, wherein said second member is fixedwith respect to said second end plate.
 8. The implant of claim 1,wherein said second member and said second end plate are integral withone another.
 9. The implant of claim 1, wherein said first and secondsaddle-shaped contact surfaces are sized and configured to permit saidfirst member to articulate with respect to said second member along saidfirst and second saddle-shaped contact surfaces through a limited angleof rotation.
 10. The implant of claim 9, wherein said angle oflimitation between said first and second members is limited by a portionof one of said first and second members contacting one of said innersides of said first and second end plates.
 11. The implant of claim 1,wherein said first member is rotatable with respect to said secondmember about two mutually skewed axes of rotation.
 12. The implant ofclaim 11, wherein said two mutually skewed axis of rotation areseparated by a distance A, said distance A being between 0.1 and 20 mm.13. The implant of claim 1, wherein said first and second bonecontacting bone contacting surfaces each include a connection elementfor engaging said first and second vertebra respectively.
 14. Anintervertebral implant for implantation between first and secondvertebra, said implant comprising: a first end plate having an innerside and a first bone contacting surface, said first bone contactingsurface being sized and configured to contact said first vertebra; asecond end plate having an inner side and a second bone contactingsurface, said second bone contacting surface being sized and configuredto contact said second vertebra; a first member having a first end and asecond end, said first end being sized and configured to contact saidfirst end plate, said second end having a first saddle-shaped contactsurface having a compound radius with at least two inflection points;and a second member having a first end and a second end, said first endbeing sized and configured to contact said second end plate, said secondend having a second saddle-shaped contact surface having a compoundradius with at least two inflection points, for contacting said firstsaddle-shaped contact surface of said first member, said first andsecond saddle-shaped contact surfaces being sized and configured topermit said first member to articulate with respect to said secondmember along said first and second saddle-shaped contact surfaces;wherein said inner side of said first end plate includes a recess, therecess having an oval shape and a circumferential shoulder and saidfirst end of said first member has a substantially circular shape and isreceivable within said recess so that said first member is slidablydisplaceable, in-situ, in a first direction with respect to said firstend plate but not in a second direction.
 15. The implant of claim 14,wherein said second member is fixed with respect to said second endplate.